Northeast Nigeria is facing a 13-year jihadist insurgency that has killed more than 40,000 people and displaced around two million more since 2009.
Fishermen told AFP that fighters rounded up dozens of fishermen from Kwatar Kaoulaha island in Diffa region from Sunday through Monday, slaughtering some and taking others hostage for disobeying a directive to leave the area.
"Many fishermen were killed and others taken hostage by Boko Haram fighters loyal to Baakura Buduma for failing to leave the area as ordered," said fisherman Kallah Sani, referring to a local Boko Haram faction chief.
Sani was among around 300 fishermen who obeyed the one-hour quit notice given by Boko Haram, leaving behind their catch and other possessions for the jihadists.
"They told us that they would take all our possessions as booty since our government has seized their money," Sani added.
"They (Boko Haram) came in two speedboats and ordered us to leave the island and not to take anything but the clothes we had on us," said a second fisherman, Anas Ibrahim.
A local Nigerian security source also confirmed the incident, saying militants had been upset by Niger's arrest of their food suppliers and a money trafficker.
"I saw 11 corpses including those of fishermen and among whom there are three Nigerians and the rest are Nigeriens," the source said.
"They say there were other deaths on the islands."
The killing of the fishermen this week underscores how deeply embedded violence has become in neighboring Niger, Chad and Cameroon, especially in the strategic Lake Chad islands where jihadists have set up hidden bases.